The Pleiades or Seven Sisters (Messier 45) is an open star cluster of about 3,000 stars containing middle-aged hot B-types located in the northern constellation of Taurus (the Bull). It is among the nearest star clusters to Earth, lying about 450 light-years away, and is the cluster most obvious to the naked eye in the night sky. The cluster is dominated by hot blue and extremely luminous stars, ranging from magnitude 2.9 to 4.2, and unrelated dust cloud in the interstellar medium that forms a faint reflection nebulosity around the brightest stars, through which the stars are currently passing. Computer simulations have shown that the Pleiades was probably formed from a compact configuration that resembled the Orion Nebula. The nine brightest stars of the Pleiades are named for the Seven Sisters of Greek mythology: Sterope, Merope, Electra, Maia, Taygeta, Celaeno, and Alcyone, along with their parents Atlas and Pleione. As a matter of perspective, the faintest stars are still 40 times brighter than our own sun would appear at a similar distance, and the brightest Pleiad, Alcyone, is 1000 times more luminous.
The Pleiades are a prominent sight in winter in the Northern Hemisphere, and have been known since antiquity to cultures all around the world, including the Celts, Māori, Aboriginal Australians, the Persians, the Arabs (known as Thurayya), the Chinese, the Japanese, the Maya, the Aztec, and the Sioux and Cherokee. Galileo Galilei was the first astronomer to view the Pleiades through a telescope. He thereby discovered that the cluster contains many stars too dim to be seen with the naked eye. He published his observations, including a sketch of the Pleiades showing 36 stars, in his treatise Sidereus Nuncius in March 1610.
The cluster core radius is about 13 light years and tidal radius is about 43 light years. The cluster contains over 1,000 statistically confirmed members, although this figure excludes unresolved binary stars. The arrangement of the brightest stars is somewhat similar to Ursa Major and Ursa Minor. The total mass contained in the cluster is estimated to be about 800 solar masses. The cluster contains many brown dwarfs, which are objects with less than 8% of the Sun’s mass, not heavy enough for nuclear fusion reactions to start in their cores and become proper stars. They may constitute up to 25% of the total population of the cluster, although they contribute less than 2% of the total mass.
Credit: NASA, Wikipedia.